1. Field of the Invention
This invention relates to E-beam lithography and particularly to a correction system for the elimination of variations of the height of the wafer or mask surfaces which tend to degrade focus (image quality) and deflection magnification and rotation (unregistered writing).
2. Prior Art
Focused electron beam lithography exposure tools are well known in the manufacture of integrated circuit technology. Such lithography exposure tools utilize a focused beam of light or electrons and maintain optimum image quality only within a very limited range of focus. This range of focus is defined as the depth of focus of the light or electron lens system. Typically, in the case of electron beam systems, the E-beam is accelerated to a high velocity and then converged into a fine beam by means of an electron lens system where it is simultaneously deflected and scanned in two dimensions utilizing a scanning coil. In such scanning systems, the E-beam does not land perpendicular to the target due to properties of the deflection system. Consequently, such scanning systems suffer from positioning errors of the spot onto the target in those situations where the distance of the target plane, vis-a-vis the optical system reference varies. This deviation in the z-direction is significant in the scanning of processed wafers. Such a wafer may have a number of chip sites, each site being at a slightly different elevation due to surface variations in the wafer, caused by diffusion, oxidation, annealing and other processing steps. In E-beam lithography tools, the wafers are in a vacuum environment and therefore vacuum chucks cannot be used to bring all the exposure sites into a flat plane.
Within the prior art, various techniques have been proposed for focusing E-beam systems. In U.S. Pat. Nos. 3,876,883; 3,901,814; and 4,199,688, techniques are disclosed to attain optimum focus by using signals produced by the electron beam itself. Such systems require the preparation of the specimens (i.e., wafers, chromium masks) with, for example, registration marks or special targets containing an aperture to which the electron beam must be directed when focus is to be optimized. This is shown specifically in the '688 patent. Other techniques use electron beam projection systems which electron-optically image a mask onto the wafer. Accordingly, focus is determined by the concept of mask projection. Such systems materially add to the complexity of the E-beam lithography process by requiring additional process steps in the preparation of specimens. Moreover, such techniques add additional steps per se into the E-beam processing sequence and therefore tend to delay the exposure cycle of the system. Throughput therefore tends to degrade.
Prior art systems employing light-optical imaging apparatus are also known, for example, as described in U.S. Pat. Nos. 3,967,109 and 4,039,824. In such systems, optimum focus is attained based on contrast generating optical properties of the object to be brought into focus, for example, slides, photographic scenes, or the like. The variation of the contrast and the image as a result of the deviation from the correct focus is utilized. In such light-optical systems step and repeat cameras are employed for projection and a sensor for determining z-error is usually incorporated in a servoloop. In such a closed loop system, if a z-error is sensed, some counter-measure, that is, movement of either the object or the focusing system, is initiated until the error is minimized.
These prior art systems also have a number of deficiencies when applied to blank wafers or chromium masks since contrast generating properties cannot generally be utilized. These devices do not have surfaces capable of generating sufficient contrast.
Light-optical step and repeat exposure systems are known which use a different auto focus method, for example, in the GEC Corporation system 3600, 3696, and 4800 cameras. Those systems utilize a light lever technique incorporating a slit light image reflecting from the surface of the target. Accordingly, the plate surface is used as a reference plane which tends to compensate for standard plate manufacturing thickness variations and tolerances together with plate surface irregularities at the photo mask image plane. A detection circuit is used to actuate a z-axis drive to change the elevation of a printer tube in which a reduction lens is mounted.
In such systems, the optical portion generally employs a light source comprising a series of alternately triggered LED's whose light is transmitted through a slit and deflected by a 45.degree. mirror onto the target. After reflection from the target surface the light passes through a piece of plane parallel glass pivoting on an axis to change the apparent location of the slit as it is reflected off the target mirror and thus change the angle of reflection to the receiver side back to the nominal angle resulting when the target plane has the nominal distance from the objective lens. The receiver side generally receives the light reflected through the slit from the surface to be scanned through a barrel lens, through another 45.degree. mirror and to an autofocus reticle. An autofocus drive mechanism is driven in either direction until the receiver senses light in both halves of the reticle. The receiver, a photo diode, receives a signal which is then amplified and sent to the servo amplifier feeding a servo motor which drives the system into a position of focus and is mechanically coupled to the pivoting plane-parallel glass plate. A rate sensor is utilized to stabilize the autofocus mechanism at a "home" position and is necessary to prevent the linear motor from continually over-driving this position. Therefore, such a system tends to dither in a continuous oscillation about the focus position. Such systems where the actuator is directly fed back to the sensor are known as closed-loop or feedback systems.
Such devices are not readily useful in E-beam systems since the E-beam cannot simply substitute for a light beam in such a system. The corresponding elements and effects are entirely different for light and for electrons. In particular, the electron beam impinging on the target surface would be scattered into a large spatial angle rather than reflected into a narrow beam. Moreover, these electrons would expose the resist. But, even if the situation could be made, the devices would not be fast enough to avoid delay in the exposure cycle. Accordingly, in E-beam exposure tools, the use of feedback techniques have not found application. There are also a number of criteria in addition to processing speed tending to limit application of prior art devices. A system for measuring the z-position of the target surface, for example, the mask or wafer, must not interfere with the E-beam or its sensors, the back scatter diodes. Also, the light which is used for the measurement technique must be in a part of the spectrum to which the resist is insensitive.
Moreover, the system must be physically compatible with the E-beam structure. It must, for example, be compatible with the high vacuum requirements in which the system operates and should not contain ferromagnetic materials or bulk conductors which tend to generate unwanted Eddy currents. Given the space requirements in a E-beam system, the z-direction focus device must be accommodated within a very limited space.
With the extreme accuracy requirements (2 .mu.m or less) the system must demonstrate high reproducible accuracy. Moreover, because of the variety of mask and wafer devices to be processed, variations in the reflectivity of the target surface must be compensated for automatically. In addition, the system must be useful in the absence of any registration marks or other aiding features on the target surface.
As indicated, given these diverse requirements, feedback devices are simply unusable. Rather, a device for measuring the z-position of the target surface must utilize some transducer technique for transforming a measurement of z-position into a correction of the focus signal, whether this be current for magnetic lenses or voltage for electrical lenses. Moreover, real time deflection correction must be accomplished utilizing computing circuitry.